1. Field of the Invention
This invention relates to a rotational angle sensor for detecting a rotational angle.
2. Description of Related Art
Conventionally, a resistor-type of sensor has been most frequently used as a rotational angle sensor. This resistor-type of sensor comprises a resistor and a bush which is rotatably in contact with-the resistor. This type of resistor is designed to be in construction and low in cost, however, it has a short life-time because it suffers abrasion through contact resistance of the brush. This abrasion problem is more critical particularly for a vehicle or the like which is driven under a severe vibration condition because the abrasion is promoted under such a condition.
In view of the foregoing, various kinds of non-contact type of rotational angle sensors have been recently used. In one of these sensors, an inductance ratio between two coils is measured to detect a rotational angle. This type of sensor is disclosed in Japanese Laid-open publication (internal laid-open application) No. 3-506071.
This publication discloses two types of sensors. One type of sensor is so designed that two coils, each having a semi-circular section, are arranged to confront each other, thereby forming a columnar coil member, and a magnetic member having a semi-arcuate section is engaged along the outer wall of the columnar member so as to be freely rotatable around the rotational center of the columnar member. On the other hand, the other sensor is so designed that two coils, each having a semi-arcuate section, are arranged to confront each other, thereby forming a cylindrical (hollow cylindrical) member, and a magnetic member having a semi-arcuate section is inserted into the inner space of the cylindrical member so as to be freely rotatable around the rotational center of the cylindrical body. Both types of sensors adopt the principle of detecting a rotational angle on the basis of a phenomenon that the inductance ratio between the two coils is variable in accordance with the rotational angle of the magnetic body.
One problem of the above conventional device for detecting the inductance ratio is that fabrication of a product is cumbersome. That is, the process of forming coils having a semi-circular or a semi-arcuate section and arranging two coils thus formed to fabricate a columnar or cylindrical (hollow cylindrical) member is more cumbersome and higher in cost in comparison with a process of forming a normal simple cylindrical coil. Further, in this device, if the positioning between the columnar or cylindrical member and the rotating magnetic member is not accurately carried out, the gap balance between these members in the rotational direction is lost, and it causes reduction of output precision, such as deterioration in linearity of the output and occurrence of offset.
Further, in the case where a magnetic shield is provided around the outer periphery of the device to remove external noises, particularly in the former device, most of the effective magnetic flux emitted from the coil is passed through the magnetic shield without being passed through the rotating magnetic member if the distance from the coil to the magnetic shield is not set to be longer than at least the distance from the coil to the rotating magnetic member, and this part of the effective magnetic flux does not contribute to the detection of a rotational angle. Accordingly, the magnetic shield must be provided away from the coil, and thus the device must be designed to be large in size.